Conventionally, in the thermal head printer, a mechanism which conveys the ink ribbon from the ribbon supply shaft on which long ink ribbon is wound through the thermal head to the ribbon winding shaft so as to wind the ink ribbon on the ink ribbon winding shaft is used. In order to maintain a constant winding tension on the ink ribbon, a slip mechanism or a mechanism for varying the torque of the winding motor according to the winding diameter of the ink ribbon wound is used in the ribbon winding shaft.
However, in a case of the slip mechanism, the winding diameter is changed with the winding amount of ribbon, which leads to a change in the tension and a lot of ribbon cannot be wound.
In a case of using a winding motor, as the mechanism for varying the torque of the winding motor according to the diameter of the wound ribbon is used, the tension may be maintained constant even if the winding diameter of ribbon becomes large, and more ribbon can be wound. However, a DC motor is used as a driving source for winding ribbon. The DC motor generates a fluctuation in torque due to the hunting characteristic at the rising time, which may affect the printing. Therefore, in order to provide tension to the ink ribbon in the start of printing, the DC motor is driven before a paper is conveyed.
However, the ink ribbon does not move because it is synchronized with the recording medium, and a load is generated in the DC motor, which is the tension. If the paper is conveyed and the ink ribbon moves, the motor load is suddenly reduced and the tension is reduced instantly. When the paper conveyance speed is high, there is a case in which the follow ability of the DC motor falls, loosening occurs in the ink ribbon. In this case, a printing failure such as a printing omission and the like occurs. Further, after a DC motor followed, a load is generated in the DC motor and the tension is increased, therefore a fluctuation in torque due to the hunting characteristic occurs. Hunting characteristic is attenuated if the constant current (load) is applied to the DC motor, but the hunting generated in the start of printing heavily affects the printing.